26.1 DNA, Chromosomes, and Genes
• When a cell is not actively dividing, its nucleus is
occupied by chromatin, which is a compact tangle
of DNA (a polymer of deoxyribonucleic acid), the
carrier of genetic information, twisted around
proteins (known as histones).
• During cell division, chromatin organizes itself
into chromosomes. Each chromosome contains a
different DNA molecule, and the DNA is
duplicated so that each new cell receives a
complete copy.
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Each DNA molecule, in turn, is made up of
many genes—individual segments of DNA that
contain the instructions that direct the
synthesis of a single polypeptide.
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26.2 Composition of Nucleic Acids
• Nucleic acid: A polymer of nucleotides.
• Nucleotide: A five-carbon sugar bonded to a
cyclic amine base and a phosphate group.
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• DNA and RNA are two types of nucleic acids.
• In RNA (ribonucleic acid) the sugar is D-ribose.
• In DNA (deoxyribonucleic acid) the sugar is 2deoxyribose. (The prefix “2-deoxy-” means that
an oxygen atom is missing from the C2 position of
ribose.)
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• Nucleoside: A five-carbon sugar bonded to a
cyclic amine base; a nucleotide with no
phosphate group.
• Nucleosides are named with the base name
modified by the ending –osine for the purine
bases and -idine for the pyrimidine bases.
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• Deoxy- is added to deoxyribose nucleosides.
• Numbers with primes are used for atoms in the sugar.
• Nucleotides are named by adding 5’-monophosphate at
the end of the name of the nucleoside.
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• For example, adenosine 5’-monophosphate (AMP)
and deoxycytidine 5’-monophosphate (dCMP).
• Nucleotides that contain ribose are classified as
ribonucleotides and those that contain 2-deoxy-Dribose are known as deoxyribonucleotides
designated by leading their abbreviations with a
lower case “d”.
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26.3 The Structure of Nucleic Acid
Chains
Nucleic acids are polymers of nucleotides. The
nucleotides are connected in DNA and RNA by
phosphate diester linkages between the –OH
group on C3’ of the sugar ring of one
nucleotide and the phosphate group on C5’ of
the next nucleotide.
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A nucleotide chain commonly has a free
phosphate group on a 5’ carbon at one end
(known as the 5’ end) and a free –OH group
on a 3’ carbon at the other end (the 3’ end).
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A nucleotide
sequence is read
starting at the 5’
end and
identifying the bases in order of
occurrence. One-letter abbreviations of
the bases are commonly used : A for
adenine, G for guanine, C for cytosine, T
for thymine, and U for uracil in RNA. The
trinucleotide at right would be
represented by T-A-G or TAG.
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26.4 Base Pairing in DNA: The WatsonCrick Model
According to the Watson–Crick model, a DNA
molecule consists of two polynucleotide strands
coiled around each other in a helical, screw like
fashion. The sugar–phosphate backbone is on the
outside of this right-handed double helix, and the
heterocyclic bases are on the inside, so that a base
on one strand points directly toward a base on the
second strand. The double helix resembles a
twisted ladder, with the sugar–phosphate
backbone making up the sides and the hydrogenbonded base pairs, the rungs.
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The two strands of the DNA double helix run
in opposite directions, one in the 5’ to 3’
direction, the other in the 3’ to 5’ direction.
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Hydrogen bonds connect the pairs of bases;
thymine with adenine, cytosine with guanine.
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The pairing of the bases along the two strands of
the DNA double helix is complementary. An A base
is always opposite a T in the other strand, a C base
is always opposite a G. This base pairing explains
why A and T occur in equal amounts in doublestranded DNA, as do C and G. To remember how
the bases pair up, note that if the symbols are
arranged in alphabetical order the outer 2 and
inner 2 pair up.
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(a) Notice that the base pairs are nearly  to the
sugar–phosphate backbones. (b) A space-filling
model of the same DNA segment. (c) An abstract
representation of the DNA double helix.
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26.5 Nucleic Acids and Heredity
• A sperm cell carrying DNA from your father
united with an egg cell carrying DNA from your
mother. Their combination produced the full
complement of chromosomes and genes that you
carry through life.
• No single cell has a lifespan equal to that of the
organism in which it is found. Therefore, every
time a cell divides, its DNA must be copied.
• Within cells, the genetic information encoded in
the DNA directs the synthesis of proteins, a
process known as the expression of DNA.
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• The duplication, transfer, and expression of
genetic information occurs as the result of
three fundamental processes: replication,
transcription, and translation.
• Replication: The process by which copies of
DNA are made when a cell divides.
• Transcription: The process by which the
information in DNA is read and used to
synthesize RNA.
• Translation: The process by which RNA directs
protein synthesis.
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26.6 Replication of DNA
• DNA replication begins in the nucleus with
partial unwinding of the double helix; this
process involves enzymes known as helicases.
• The unwinding occurs simultaneously in many
specific locations known as origins of
replication. The DNA strands separate,
exposing the bases. These branch points,
called replication forks, provide a “bubble”
into which the replication process can begin.
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